1. Field of the Invention
The present invention relates to sensors for detecting the presence of a fluid, automatic systems for actuating pumps in response to detecting a fluid level and sensors mounted in the bottom of a boat bilge tank activating a bilge pump when the bilge fluid level reaches a preset distance above the bottom of the bilge tank.
2. Discussion of the Prior Art
In the past, bilge pumps have been activated manually or by mechanical float type switches with mercury or point contacts to complete an electrical circuit activating a pump. Pressure switches have also been used. These prior art switches worked adequately when initially installed. Over time, however, bilge debris and other sources of contamination often prevented the mechanical components from moving as intended, causing switch failure. In addition, prior art bilge pump activation switches typically wore out several times during the life of a boat and, being located in a boat's nether regions, were difficult to access for repair and replacement.
Many fluid level or fluid proximity detectors of the prior art employed electrical switches actuated when a conductor, such as a body of water, moved into close proximity to the detector or sensor. U.S. Pat. Nos. 3,588,859; 3,665,300; 4,800,755 and 4,875,497 disclose such detectors. U.S. Pat. No. 5,017,909, discloses a proximity detector used as a liquid level detector for receptacles in vehicles.
Other applications for liquid level detectors included bilge-pumping systems for ships. A bilge pumping system must be activated before the accumulated water reaches an excessive level. Prior art mechanisms for detection of an excessive bilge water level employed mechanical floatation systems, causing a switch to be actuated whenever the water reached such an undesired level. Bilge fluid or water eventually renders mechanical level sensing systems inoperative in part because bilge fluid can contain many forms of corrosive waste. Replacing failed parts a bilge level sensing system can be very expensive and troublesome, since a skilled technician must enter the bilge to perform the work.
Many electronic proximity detection systems have been proposed in searching for a solution to this messy, expensive problem. By way of example, Smith et al (U.S. Pat. No. 4,881,873) discloses a capacitive level sensor for a bilge pump including a sensor plate 40 positioned in a bilge at a position selected for pump actuation. The bilge water is sensed as a dielectric, in a manner of speaking, and so the sensor is susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the area around the sensor, and sloshing bilge water is likely to cause the bilge control to actuate when the bilge level does not require pumping.
Gibb (U.S. Pat. No. 5,287,086) also discloses a capacitive level sensor for a bilge pump including a capacitive sensor plate 79 positioned in a bilge at a position selected for pump actuation. The sensor is contained within a sealed housing 32 to keep bilge water away from the sensor and other circuitry. Here again, bilge water is sensed as a dielectric, in a manner of speaking, and so the sensor is susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the area around the sensor, and sloshing bilge water is likely to cause the bilge control to actuate when the bilge level does not require pumping.
Santiago (U.S. Pat. No. 4,766,329) discloses a solid-state two level sensor for a bilge pump including a high water level probe and a low water level probe, both positioned in a bilge at positions selected for pump actuation. The probes are in contact with the bilge water, and so the probe sensors are susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the probes.
Farr (U.S. Pat. No. 5,238,369) discloses a system for liquid level control including upper and lower capacitive level sensors 10, 18 having positions selected for pump actuation. This reference is silent on the need to keep bilge water away from the sensors, but the bilge water is sensed as a dielectric, in a manner of speaking, and so the sensor is susceptible to false alarms or missed detections once the contamination accompanying bilge inflow has accumulated in the bilge and contaminated the area around the sensors.
The applicant has licensed a Field Effect sensor patent to Caldwell (U.S. Pat. No. 5,594,222) on a “touch sensor” used to detect whether a user presses a virtual button; this sensor is referred to as a “touch sensor.” While the patent discloses the electromagnetic properties of Field Effect “touch” sensing, it is silent on how such technology might be employed in a sensor system for detecting a fluid/air interface or for automated bilge pump actuation.
There is a need, therefore, for a system for sensing liquid level and liquid level control that overcomes the problems with prior art sensors and systems, permitting installation of a reliable, inexpensive fluid level sensing system which is unlikely to require maintenance or cleaning in the bilge. It would be highly desirable to have a new and improved proximity detection system which is highly reliable and relatively inexpensive to manufacture. Such a proximity detection system should be highly sensitive and possess a wide range of applications.